The human spine is made up of a column of thirty-three bones and their adjoining structures. The bodies of these vertebrae are connected by anterior and posterior ligaments and by discs of fibrocartilage generally known as intervertebral discs. These discs are positioned between opposite faces of adjacent vertebral bodies. This column of vertebrae and intervertebral discs forms a central axis that supports the head and torso. These vertebrae also enclose an opening through which the spinal cord passes.
One of the most costly health problems in society involves back pain and pathology of the spine. These problems can affect individuals of all ages and can result in great suffering to victims. Back pain can be caused by several factors such as congenital deformities, traumatic injuries, degenerative changes to the spine, and the like. Such changes can cause painful excessive motion, or collapse of a motion segment resulting in the contraction of the spinal canal and compression of the neural structures, causing debilitating pain, paralysis or both, which in turn can result in nerve root compression or spinal stenosis.
Nerve conduction disorders can also be associated with intervertebral discs or the vertebrae themselves. One such condition is herniation of the intervertebral disc, in which a small amount of tissue protrudes from the sides of the disc into the foramen to compress the spinal cord. A second common condition involves the development of small bone spurs, termed osteophytes, along the posterior surface of the vertebral body, again impinging on the spinal cord.
Upon identification of these abnormalities, surgery may be required to correct the problem. For those problems associated with the formation of osteophytes or herniations of the intervertebral disc, one such surgical procedure is intervertebral discectomy. In this procedure, the involved vertebrae are exposed and the intervertebral disc is removed, thus removing the offending tissue or providing access for the removal of the bone osteophytes. A second procedure, termed a spinal fusion, may then be required to fix the vertebrae together to prevent movement and maintain a space originally occupied by the intervertebral disc. Although this procedure may result in some minor loss and flexibility in the spine due to the relatively large number of vertebrae, the minor loss of mobility is typically acceptable.
For the replacement of vertebra of the human spinal column, for the distraction of the spinal column, for the stabilization of the vertebrae and likewise, it is known to apply pedicle screws. The pedicle screw is screwed into the pedicle of the vertebra and the head of the pedicle screw is connected to suitable provisions, for example to a stabilizing system, to distraction rods, etc. During the treatment of the spine, the pedicle screw is generally first rotated into the pedicle. Subsequently, the insertion of the rod is effected.
A standard pedicle screw assembly comprises a screw having an externally threaded stem having in turn a head provided with parts allowing it to be secured to one end of a distraction rod. Typically two such pedicle screws are inserted into respective vertebrae and are secured to a rod to distract and/or stabilize a spinal column after, for instance, a disk operation. One commonly used pedicle screw is disclosed in German Patent No. 4,107,480, which is incorporated herein by reference, and includes a head that has a pair of outwardly projecting parallel ridges with overhanging inner edges. A cap formed with a pair of complementary inwardly open slots fits with these ridges. The pedicle screw is threaded into the vertebrae, an end of the rod is fitted to its outer end, the cap is then slid transverse to the pedicle screw axis and parallel to the rod, over the rod to capture it, and finally a cap screw threaded into the cap and tightened to press the rod down against the head of the pedicle screw and thereby fix the rod, cap, and screw together. Many other pedicle screw designs have been developed to simplify the insertion of the pedicle screw into the pedicle, and/or to reduce damage to the pedicle screw and/or the pedicle during surgery. Some of these pedicle screw designs are disclosed in U.S. Pat. Nos. 5,882,350; 5,989,254; 5,997,539; 6,004,322; 6,004,349; 6,017,344; 6,053,917; 6,056,753; 6,083,227; 6,113,601; 6,183,472; 6,224,596; 6,368,319; 6,375,657; and 6,402,752; and the patents cited and disclosed in such patents. All these designs of pedicle screws are incorporated herein by reference.
After the pedicle screw is inserted in the pedicle, the bone around the pedicle screw must heal to properly secure the pedicle screw in the bone. Any infection that occurs around the pedicle screw can slow the healing process and/or damage the bone around the pedicle screw thereby weakening the connection between the bone and pedicle screw. Typically, a patient is given antibiotics for several days after the surgery to reduce the occurrence of infection about the pedicle screw. The patient may also receive electrical stimulation during surgery to promote the healing process of the bone about the pedicle screw. Both of these techniques have improved the post-operative success of the surgical procedure; however, improved success rates are still needed.